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happyphysics.com Physics Lecture Resources Prof. Mineesh Gulati Head-Physics Wing Happy Model Hr. Sec. School, Udhampur, J&K Website: happyphysics.com

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Ch 26 Direct- Current Circuits © 2005 Pearson Education

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26.1 Resistors in Series and Parallel resistors in series © 2005 Pearson Education I

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resistors in parallel © 2005 Pearson Education I1I1 I2I2 I3I3

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26.2 Kirchhoff’s Rules Kirchhoff’s junction rule: The algebraic sum of the currents into any junction is zero. Kirchhoff’s loop rule: The algebraic sum of the potential differences in any loop, including those associated with emfs and those of resistive elements, must equal zero. © 2005 Pearson Education

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Junctions

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26.3 Electrical Measuring Instruments © 2005 Pearson Education d’Arsonval galvanometer

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© 2005 Pearson Education Shunt resistor Ammeter Voltmeter

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Example 26.8 What shunt resistance is required to make the 1.00mA, 20.0Ω meter described above into an ammeter with a range of 0A to 50.0mA? What shunt resistance is required to make the 1.00mA, 20.0Ω meter described above into an ammeter with a range of 0A to 50.0mA? © 2005 Pearson Education ANS:

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© 2005 Pearson Education Ohmmeters

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© 2005 Pearson Education Potentiometer

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26.4 R-C Circuits © 2005 Pearson Education

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Charging Capacitor

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© 2005 Pearson Education Discharging a Capacitor

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© 2005 Pearson Education

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26.5 Power Distribution Systems © 2005 Pearson Education

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When several resistors R1, R2, R3,… are connected in series, the equivalent resistance Req is the sum of the in a series connection. When several resistors are connected in parallel, the reciprocal of the equivalent resistance Req is the sum of the reciprocals of the individual resistances. All resistors in a parallel connection have the same potential difference between their terminals. (see Examples 26.1 and 26.2) © 2005 Pearson Education

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Kirchhoff’s junction rule is based on conservation of charge. It states that the algebraic sum of the currents into any junction must be zero. Kirchhoff’s loop rule is based on conservation of energy and the conservative nature of electrostatic fields. It states that the algebraic sum of potential differences around any loop must be zero. Careful use of consistent sign rules is essential in applying Kirchhoff’s rules. (See Examples 26.3 through Example 26.7) © 2005 Pearson Education

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In a d’ Arsonval galvanometer, the deflection is proportional to the current in the coil. For a larger current range, a shut resistor is added, so some of the current bypasses the meter coil. Such an instrument is called an ammeter. If the coil and any additional series resistance included obey Ohm’s law, the meter can also be calibrated to read potential difference or voltage. The instrument is then called a voltmeter. A good ammeter has very low resistance; a good voltmeter has very high resistance. © 2005 Pearson Education

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When a capacitor is charged by a battery in series with a resistor, the current and capacitor charge are not constant. The charge approaches its final value asymptotically and the current approaches zero asymptotically. The charge and current in the circuit are given by Eqs. (26.12) and (26.13). After a time τ=RC, the charge has approached within 1/e of its final value. This time is called the time constant or relaxation time of the circuit. When the capacitor discharges, the charge and current are given as functions of time by Eqs. (26.16) and (26.17). The time constant is the same for charging and discharging. (See Examples and 26.13) © 2005 Pearson Education

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In household wiring systems, the various electrical devices are connected in parallel across the power line, which consists of a pair of conductors, one “hot” and the other “neutral.” An additional “ground” wire is included for safety. The maximum permissible current in a circuit is determined by the size of the wires and the maximum temperature they can tolerate. Protection against excessive current and the resulting fore hazard is provided by fuses or circuit breakers. (See Example 26.14) © 2005 Pearson Education

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